Can you use DEF fluid after it’s been opened?

Diesel exhaust fluid (DEF) is a key component of modern diesel engine emissions systems. Once opened, DEF has a limited shelf life. However, with proper storage it can often be used well past the manufacturer’s recommended discard date. The key factors are avoiding contamination and temperature extremes.

What is DEF and why is it used?

DEF is an aqueous urea solution made with 32.5% high-purity urea and 67.5% deionized water. It is used in selective catalytic reduction (SCR) systems to lower NOx emissions from diesel engines to meet EPA regulations.

When injected into the exhaust stream, the DEF breaks down into ammonia and reacts with a catalyst to convert NOx into harmless nitrogen gas and water vapor. Most modern diesel engines need DEF for emissions compliance.

DEF and SCR emissions systems

SCR systems inject small amounts of DEF into the exhaust upstream of a catalyst. The heat of the exhaust gas causes the urea to decompose into ammonia. This ammonia reacts with the SCR catalyst to convert NOx emissions into nitrogen and water.

Most diesel vehicles and equipment produced since 2010 utilize SCR and require DEF. Tier 4 Final EPA regulations went into effect in 2014, requiring still lower NOx emissions from off-highway diesel engines.

Does DEF expire?

DEF has a limited shelf life, especially once a container has been opened and exposed to the atmosphere. Two processes contribute to the degradation of DEF over time:

  • Hydrolysis – Urea gradually reacts with water to produce ammonia.
  • Contamination – Bacteria and debris can enter opened containers and decompose urea into byproducts.

These effects accelerate at higher temperatures. Direct sunlight and high heat cause DEF to degrade faster. The ideal storage temperature is below 77°F (25°C).

Manufacturer shelf life specifications

Most major DEF suppliers provide a recommended shelf life for unopened containers:

  • Cummins – 2 years
  • Shell Rotella – 2 years
  • PEAK – 2 years
  • Valvoline – 3 years

Once opened, the shelf life decreases considerably. Rotella specifies a shelf life of only 6 months after opening under proper storage conditions. Cummins suggests discarding any remaining DEF within 3 months of opening a container.

Can you use DEF after it expires?

While DEF degrades over time after the recommended discard date, it does not immediately become unusable. If properly stored, opened DEF can often last 6-12 months past the manufacturer’s discard date before falling out of ISO 22241 specifications.

Testing aged DEF

The ISO 22241 standard provides quality benchmarks for DEF, including:

  • Urea content – 31.8-33.2%
  • Alkalinity – 0.2% max.
  • Insoluble solids – 0.08% max.
  • Phosphates – 0.5 ppm max.
  • Calcium – 0.5 ppm max.
  • Iron – 0.5 ppm max.
  • Copper – 0.2 ppm max.
  • Zinc – 0.2 ppm max.
  • Chromium – 0.2 ppm max.
  • Nickel – 0.2 ppm max.
  • Aluminum – 0.5 ppm max.
  • Magnesium – 0.5 ppm max.
  • Sodium – 0.5 ppm max.
  • Potassium – 0.5 ppm max.
  • Ammonium – 0.2% max.

Testing can determine whether stored DEF remains within specification after the manufacturer’s shelf life. Some fleets and owner/operators have DEF tested annually to determine if their supply remains usable.

Appearance and smell

Visual and olfactory inspection provides a basic gauge of DEF quality. Undegraded DEF is clear, colorless, and odorless. If expired DEF appears cloudy, discolored, or has a sharp, pungent odor, it is likely well past its usable life.

What happens if you put bad DEF in a diesel truck?

Using degraded or contaminated DEF can lead to a range of issues:

  • SCR system damage – Contaminants and byproducts from broken-down DEF can cause mechanical problems in SCR system components like injectors, sensors, and catalyst substrate.
  • NOx emissions increase – As DEF quality falls, NOx conversion efficiency declines, leading to higher tailpipe NOx.
  • Emissions system alerts – Out-of-spec DEF will trigger diagnostic trouble codes and Check Engine lights related to the emissions system.
  • Power derates – To encourage operators to fix the issue, some engines will go into a power-limited “limp home” mode in response to bad DEF.

Problems related to bad DEF are usually gradual. Initially, the engine computer may adapt to moderately degraded DEF. But as quality falls further out of spec, derates, operational issues, and even irreversible SCR damage can occur.

Avoiding SCR system damage

If a diesel truck has been filled with severely degraded or contaminated DEF, the SCR system should be purged by a technician before adding new DEF. Trying to dilute bad DEF by adding new fluid on top can be worse than not acting at all.

Prompt action also helps prevent irreversible catalyst damage. Technicians have procedures to flush contaminated DEF and reset the emissions system.

Best practices for storing DEF

Proper storage is key to maximizing the usable life of DEF after opening. Recommended storage procedures include:

  • Use only EPA/CARB-approved containers designed for DEF
  • Keep stored DEF cool, ideally below 77°F (25°C)
  • Avoid direct sunlight and heat sources
  • Store indoors or in cool shade if possible
  • Keep containers sealed and free of contamination
  • Avoid cross-contaminating DEF containers and equipment
  • Check containers and supply tanks regularly for appearance and odor changes
  • Consider annual DEF testing to check for degradation
  • Label containers with opening date to track shelf life

Practicing first in/first out usage, limiting inventory, and not overbuying DEF also helps prevent waste from shelf life expiration before opened containers can be used.

Avoiding contamination

Once opened, DEF is vulnerable to contamination from dirt, bacteria, other fluids, and environmental debris. Using dedicated DEF-only containers, filling equipment, and storage areas helps prevent cross-contamination.

Partially used containers should be re-sealed immediately. Supplies should be checked regularly for signs of contamination like unusual color, odor, clarity changes, or growth of biological agents.

Can contaminated DEF be restored?

Severely contaminated DEF likely needs to be disposed of and replaced. However, if DEF shows early signs of contamination, prompt action may restore its usable life.

Methods that may rehabilitate mildly degraded DEF include:

  • Filtering – Removing particulate through a fine diesel fuel filter can clean up hazy or dirty DEF.
  • Straining – Pouring through a paint filter or coffee filter catches larger particles.
  • Settling – Letting the DEF sit undisturbed for 1-2 days allows some contaminants to settle out.
  • Dilution – Mixing with an equal amount of new DEF can remediate early degradation.
  • Heating – Gently warming to 105-110°F helps dissolve some precipitates.
  • Ultrasonic cleaning – Using ultrasonic vibration breaks down and disperses particulates.
  • Centrifuging – A centrifuge can separating insolubles from liquid DEF.

Trying to salvage already degraded DEF may not be worth the effort. But mildly contaminated fluid can possibly be restored by DIY methods.

When to cut your losses

Once DEF shows obvious signs of precipitation, darkening, or foul odor, it is unlikely to be economical or practical to recondition. At that point it is best to dispose of it properly.

Technician cleaning services are available for badly contaminated bulk DEF tanks. But heavily degraded drummed supplies are better off recycled or disposed rather than attempting restoration.

Alternative uses for out of spec DEF

Because urea is a useful chemical, some applications can benefit from degraded DEF that no longer meets ISO 22241 as an emissions fluid. Potential alternative uses include:

  • Fertilizer – The urea, water, and contaminants like potassium and phosphates in old DEF make an excellent, if smelly, lawn or garden fertilizer.
  • NOx reductant – Out-of-spec DEF still converts some NOx to nitrogen and water when injected into combustion exhaust streams.
  • Diesel fuel additive – Diluted DEF has been reported to offer cetane improvements and combustion knock reduction when added to diesel in small percentages – though this is not recommended.
  • Animal repellent – The strong ammonia odor of degraded DEF helps deter nuisance species like deer and pets from areas where it is spread.

Before using expired DEF for alternative purposes, it is advisable to have it tested. While urea concentrations fall over time, levels of accumulated contaminants may make re-purposing inadvisable.

Legal considerations

It is illegal under EPA regulations to tamper with emissions controls. Using degraded DEF as a diesel additive or injecting into exhaust to avoid buying new supply violates federal law. Fines of up to $37,500 per day can be imposed for tampering.

Operators using out-of-spec DEF for legitimate alternative purposes should exercise safety and common sense.

Disposing of outdated or contaminated DEF

DEF has a high biological oxygen demand. Spills can deplete oxygen in lakes, rivers and groundwater, harming ecosystems. Proper disposal is important.

Recommended disposal methods include:

  • Recycling – Some community household hazardous waste facilities accept DEF for urea recycling.
  • Wastewater treatment – Small volumes can be diluted with water at least 10:1 and disposed to sanitary sewer where allowed by ordinance.
  • Incineration – Hazardous waste handlers can combust concentrated DEF as an aqueous waste.
  • Fertilizer – Applying degraded DEF to turf or farm fields puts nitrogen and phosphates to beneficial use.

DEF is generally considered non-hazardous waste unless containing heavy metal contaminants above regulated thresholds. Never discharge directly into storm drains, septic systems, or surface waters.

Avoiding environmental contamination

Spills of any amount should be contained and absorbed. Soil or pavement saturated with DEF may need removal. Runoff into waterways must be prevented.

Minimizing surplus DEF inventory through proper purchasing and rotation helps reduce disposal volumes. Keeping DEF sealed in approved containers controls accidental releases.

Typical DEF Shelf Life by Container Size
Container Size Shelf Life Unopened Shelf Life Opened
2.5 Gallons 2 years 6 months
5 Gallons 2 years 6 months
55 Gallons 2 years 12 months
Bulk Storage 2 years 12 months

Larger containers and tanks provide more usable time once opened, but proper storage and testing is still important to maximize shelf life.


Diesel exhaust fluid has a limited shelf life, especially after containers have been opened. However, with proper storage and handling, DEF can often be used for 6-12 months past manufacturer expiration dates before falling out of specification.

The keys are keeping DEF cool, avoiding contamination, and testing to identify degradation. Fleet managers should practice first-in-first-out usage, label containers, and watch for signs of aging DEF.

Alternative uses may be possible for out of spec DEF, but consult hazardous waste regulations. Disposal is best done through recycling, wastewater treatment, or hazardous waste incineration when possible.

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